Retirement counts as a job-based coverage loss
Voluntarily retiring and losing your employer plan opens the same 60-day Special Enrollment Period as an involuntary job loss -- you can enroll in a Marketplace plan starting up to 60 days before your coverage ends, or within 60 days after.
COBRA vs. Marketplace during the bridge years
COBRA lets you keep your exact employer plan, generally at the full premium plus up to a 2% administrative fee -- often expensive. A Marketplace plan may cost less once a premium tax credit is applied, especially since retirement income is frequently lower than working income. The COBRA vs. Marketplace comparison tool gives you a real number to compare against your actual COBRA quote.
The bridge has an end date: Medicare
Once you turn 65, Medicare's own enrollment rules take over -- a different system from the ACA Marketplace. Use the Medicare Enrollment Calculator as that date approaches to see your exact 7-month Initial Enrollment Period window.